Alkanolamine sweetening units are used for the removal of H.sub.2 S and CO.sub.2 from natural gases, enhanced oil recovery gases, refinery hydrodesulfurizer recycle gases, FCCU and Coker gas plant tail gases, LPG streams, and Claus sulfur recovery tail gases. The alkanolamines (AAmines) commonly used are ethanolamine, diethanolamine, methyl diethanolamine, diisopropanol amine, and triethanol amine. These compounds are weak bases in aqueous solution. When solutions of alkanolamines are contacted in packed, sieve plate, bubble cap, or valve tray columns with streams containing H.sub.2 S and CO.sub.2, the H.sub.2 S and CO.sub.2 dissolve into the alkanolamine solution. The following chemical reactions then take place: EQU H.sub.2 S+AAmine=AAmine H.sup.+ +HS.sup.- EQU H.sub.2 O+CO.sub.2 +AAmine=AAmineH.sup.+ +HCO.sub.3.sup.-
General Eqn.: Acid Gases+Alkanolamine=Alkanolamine Salts of Acid Gases
The solution of water, unreacted alkanolamine, and alkanolamine salts is subjected to steam stripping to reverse the above reaction and remove H.sub.2 S and CO.sub.2 from the alkanolamine. The H.sub.2 S and CO.sub.2 removed from the alkanolamine can then be processed by Claus sulfur recovery, incineration, fertilizer manufacture, or other means.
H.sub.2 S and CO.sub.2 are not the only gases in the above referred to streams which form weak acids when dissolved in water. Other such acid gases, as they are commonly called, that may appear in gas streams treated with alkanolamine include SO.sub.2, COS, or HCN. These gases also undergo the same reactions as H.sub.2 S and CO.sub.2 to form alkanolamine salts. These salts, however, cannot be removed by steam stripping as are H.sub.2 S and CO.sub.2 salts. Thus, they remain and accumulate in the system.
Another problem is presented if oxygen gets into the alkanolamine system. Oxidation of acid gas conjugate base anions leads to the formation of other alkanolamine salts, most commonly salts of thiosulfate S.sub.2 O.sub.3.sup.= and sulfate SO.sub.4.sup.=. Alkanolamine salts are also formed with thiocyanate (SCN.sup.-) and chloride (Cl.sup.- These salts also cannot be regenerated by steam stripping.
Alkanolamine salts which cannot be heat regenerated, called heat-stable salts, reduce the effectiveness of alkanolamine treating. The alkanolamine is protonated and cannot react with H.sub.2 S and CO.sub.2, which dissolve into the solution. Also, accumulated alkanolamine salts are known to cause corrosion in carbon steel equipment which is normally used in amine systems. These salts are also known to cause foaming problems which further decreases treating capacity.
One procedure used to deprotonate the alkanolamine, so it can react with H.sub.2 S and CO.sub.2, is to add an alkali metal hydroxide, such as NaOH, to the amine solution. The deprotonated alkanolamine then can then be returned to H.sub.2 S and CO.sub.2 removal service. However, the sodium salts of the anions of the heat-stable salts are also heat stable, are difficult to remove, and thus accumulate in the alkanolamine solution with attendant corrosion and foaming problems.
The alkanolamine solution containing alkali metal salts of anions which form heat-stable salts with such alkanolamine may be regenerated by contacting it with a cation exchange resin whereby alkali metal ions are removed from the solution and thereafter contacting the solution with a basic anion exchange resin to remove the heat stable anions from the solution. The anion exchange resin is thereafter regenerated with a dilute sodium hydroxide, and the cation exchange resin is regenerated with a dilute mineral acid. The two resins are flushed with water before and after the regeneration procedures.
In another procedure, described in U.S. Pat. No. 2,797,188, alkanolamine solution containing heat-stable salts of the alkanolamines with thiocyanates and formates is regenerated in a two-stage process. In the first stage, the solution is contacted with a strong base anion exchange resin which has a high affinity for the thiocyanate anions. The solution leaving the first stage, which is substantially reduced and/or free of thiocyanate anions is then contacted in the second stage with the same type of ion exchange resin wherein formate anions are substantially removed. The process is continued until breakthrough of thiocyanate anions from the first stage resin when the process is halted. Regeneration of the first stage resin is carried out by contacting the resin in counter-current flow with a dilute aqueous solution of a soluble salt comprising a polyvalent anion, e.g., sodium sulfate. After the resin has been converted to the sulfate form with commensurate removal of thiocyanate anions, the resin is contacted with aqueous alkali, e.g., sodium hydroxide to remove the sulfate and is then flushed with water to complete the regeneration.
The second stage resin is regenerated in a counterflow by contacting the resin with aqueous alkali hydroxide to remove the formate anions followed by a water wash to complete the regeneration.
It is apparent that conjugate base anions of acids are present during various stages of the alkanolamine treating process and also during the procedures carried out to reclaim spent alkanolamine. It would be desirable to have a process for determining the concentration and type of anions present in the alkanolamine solution at various stages of the treating process to reduce costs associated with undercirculation, high corrositivity and poor treating of amine streams. It would also be desirable to monitor alkanolamine reclamation processes.